The subject matter disclosed herein relates generally to light curtains and photoelectric barriers and, more specifically, to safety light curtains for monitoring a protective field and, in particular, to such light curtains which comprise a plurality of optoelectronic components interconnected by a communication bus. In particular, the present invention relates to a method for synchronizing the optical units of a photoelectric barrier and to such a photoelectric barrier.
Generally, photoelectric barriers, also referred to as light curtains or light grids, detect the movement or intrusion of objects into guarded zones, and more particularly, provide protection for human operators who are working with machines or other industrial equipment.
Light curtains employing infrared or visible light beams are used to provide operator safety in a variety of industrial applications. Light curtains typically are employed for operator protection around machinery, such as punch presses, brakes, molding machines, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like. Conventional light curtains typically employ light emitting diodes (LED) mounted at spaced positions along a transmitter bar at one side of the guard zone and phototransistors (PT), photodiodes or photoreceivers mounted along a receiver bar at the opposite side of the zone. The LEDs transmit modulated infrared light beams along separate parallel channels to the PTs at the receiver bar. If one or more beams are blocked by an opaque object, such as the operator's arm, a control circuit shuts the machine down, prevents the machine from cycling, or otherwise safeguards the area.
Usually, safety light curtains comprise two optical units (also called bars, sticks, edges or strips), which are formed as two different constructional units, one of the optical units having the functionality of an emitter and one of a receiver. This dedicated architecture of an emitter and receiver, however, has several drawbacks.
Firstly, the fabrication costs are high, because each type of optical unit has to be fabricated differently. Further, due to the fact that the optical communication is only unidirectional, i.e. from the sender to the receiver, the optical synchronization may be difficult and a transmission of communication information is possible only in one direction.
it has already been proposed to locate receivers and transmitters on each of the first and second optical units, as this is described in the European patent EP 1 870734 B1. Here, the light curtain has two identical transmitting/receiving strips, to which transmitting and receiving units are fixed. The transmitting/receiving strips are placed opposite to each other with a protective field being formed between the strips. The transmitting/receiving strips are identically formed in control and evaluation units. The control and evaluation units have safety outputs, which are formed together as a switching channel. An identical power supply is provided for all the strips.
Furthermore, it is known from EP 2511737 A1 to provide a modular light curtain and optical unit for such a light curtain.
When operating a photoelectric barrier, there is always the need of a tight synchronization of the optical units forming the light curtain in respect to each other. For establishing a synchronized operation two different cases have to be distinguished. Firstly, after the initial startup or after a complete interruption of the communication between the two parts of the light curtain, the system is completely out of synchronization and the two optical units have to find each other, and if the emitter on one of the units sends a synchronization signal, the receiver on the other optical unit does not necessarily have to be prepared to receive a signal from the opposing emitter.
On the other hand, also during normal operation when the system is synchronized by and large, this synchronization gets lost in time due to slightly different clock frequencies and propagation times within the optical units. Therefore, the two optical units also have to be resynchronized by some few microseconds during normal operation.
Furthermore, photoelectric barriers often have the function of a so-called blanking, meaning that one or two radiation beams can be disabled in order to allow larger objects to pass through the sensing field without causing an alarm signal. Effectively, this blanking changes intermediately the minimum object resolution of the light curtain in order to for instance allow for supply material to enter the protected area. Apart from the so-called fixed channel blanking, where only pre-defined fixed beams can be switched off, the so-called floating blanking allows the disabling of up to two light curtain beams at any location within the sensing field.
In known systems, a drawback can be seen in the fact that only those regions are accessible for a floating blanking which are not needed for the synchronization.
From U.S. Pat. No. 8,058,605 B2 an optoelectronic sensor is known that comprises a first and a second sensor part each being composed of a plurality of transceiving modules which exchange modulated beams for reducing the effects of optical disturbances for instance due to light reflection or incident sunlight. The optical communication protocol between the two sensor parts can be used for synchronizing the sensor parts to each other. However, according to this document only the transmitting and receiving modules of the sensor parts situated directly opposite one another can communicate optically with one another by means of transmitted beams.
Thus, it would be desirable to provide a light curtain and a method for synchronizing at least two optical modules with each other, which reduces the expenditure during assembly and also complies with the requirements for a design for testability and maintenance.